Since the decline in use of natural stone
at the beginning of the twentieth century most domestic and
small commercial properties in Cornwall and South Devon have
been built with concrete blocks or mass concrete. Until the
mid-1950’s concrete blocks were often locally made, sometimes
by individual builders, and shuttered concrete was mixed on
site.
Before the second world war concrete products were rarely
transported more than 20 km. Because concrete was made where
it was needed and as transport was difficult and costly, aggregates
were sought locally. Now all blocks are factory produced and
mass concrete is supplied ready-mixed from facilities where
manufacture is strictly controlled.
In many parts of Cornwall
and South Devon ample supplies of cheap and often suitably graded
aggregate were available as waste materials from the region’s
metalliferous mining industry. The use of mining and, more particularly,
ore processing wastes as aggregates, is central to the problem
of accelerated concrete degradation in the region. It is widely
called “the mundic block problem”.
Mundic is an old Cornish word for the common sulphide mineral
pyrite. Accelerated deterioration is generally associated with
the in situ oxidation of pyrite (and other sulphide minerals)
in mine waste aggregates and sulphuric acid attack on the cement.
Deterioration is occasionally so severe that concrete becomes
structurally unsafe and some properties have had to be demolished.
In 1994 the Royal Institute
of Chartered Surveyors (RICS) published guidelines for the testing
of concrete block and shuttered mass concrete from properties
built prior to the mid-1950's. This original guidance note has
since been up-dated and amended. The testing regime so established
has been designed to classify the concrete, primarily to satisfy
lending institutions that a property is fit to mortgage on normal
terms.
